THRIFT-1742 Optionally implement hashcode and equals in c#

Patch: Carl Yeksigian
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/thrift
THRIFT-1955 Union Type writer generated in C# does not WriteStructBegin

Patch: Carl Yeksigian
THRIFT-1949: WP7 build broken